1. Field of the Invention
This invention relates generally to the field of acoustic sensors and more particularly to a novel hydrophone and method of making the same which may be used under great hydrostatic pressure and under severe hydrodynamic conditions.
2. Description of the Related Art
Piezoelectric hydrophones of various configurations have been used in a variety of applications. In geophysical exploration, arrays of hydrophones are used to detect seismic shock waves from the earth's substrata in response to induced shock waves at known locations on the earth. Hydrophones also are used in boreholes to conduct vertical seismic surveys and for a variety of other applications. Acoustic pressure variations across the hydrophone produce electrical signals representative of the acoustic pressure, which are processed for desired applications.
Piezoelectric hydrophones typically contain a piezoelectric material as an active element which produces electrical signals when subjected to acoustic pressures. Ceramic materials such as barium titane or lead zirconate titane have been used in various configurations as one class of piezoelectric materials in hydrophones. U.S. Pat. No. 4,092,628 discloses the use of a thin ceramic wafer that operates in the bender mode. U.S. Pat. No. 4,525,645 shows a unit shaped as a right cylinder that operates in the radial mode. Ceramic materials are brittle and tend to shatter in the presence of a severe shock such as that produced by an explosive charge or air gun commonly employed for conducting seismic surveys over water-covered areas.
Most hydrophones have a depth limit. An excessive overpressure can cause the active element to bend beyond its elastic limit, resulting in signal distortion and ultimate failure of the hydrophone. In the ceramic wafer type hydrophones, an internal stop is sometimes provided to prevent excessive bending of the element. The wafer, however, tends to develop a permanent deformation that degrades the output signal.
Polyvinylidene fluoride ("PVDF") has been used as another class of piezoelectric material in hydrophones. One such material is available under the tradename KYNAR from AMP corporation. The PVDF material is useful as a hydrophone active element because its acoustic impedance is close to that of water and the acoustic wavefields do not produce spurious reflections and diffractions as they do when encountering ceramic piezoelectric elements. The output signals of the PVDF element are many times greater than the signal output of a ceramic material. Also, PVDF material is readily available in various sheet sizes and a wide range of thickness. Such PVDF material may be readily shaped and cut to fit the intended use. Prior to use, the PVDF material is poled or activated in the thickness direction by application of a high electric field at an elevated temperature for a requisite time period. Conductive metal electrodes are evaporated on the opposite sides of the PVDF film as with the ceramic materials.
An external mechanical force applied to the PVDF film results in a compressive or tensile force strain. The PVDF film develops an open circuit voltage (electrical charge) proportional to the changes in the mechanical stress or strain. The charge developed diminishes with time, depending upon the dielectric constant of the film and the impedance of the connected circuitry. By convention, the polarization axis is the thickness axis. Tensile stress may take place along either the longitudinal axis or the width axis.
U.S. Pat. No. 4,653,036 teaches the use of a PVDF membrane stretched over a hoop ring. A metallic backing is attached to the back of the ring and a void between the film and the backing is filled with an elastomer such as silicone. The device operates in the bender mode. U.S. Pat. No. 4,789,971 shows the use of a voided slab of PVDF material sandwiched between a pair of electrodes. A bilaminar construction is also disclosed. A preamplifier is included in the assembly. The transducer operates in the thickness-compressive mode.
A hydrophone array shown in U.S. Pat. No. 4,805,157 consists of multiple layers of PVDF material symmetrically disposed around a stiffener for prevention of flexural stresses. The axis of maximum sensitivity is in the direction transverse to the plane of the layers. This sensor is sensitive to compressive stress.
U.S. Pat. No. 5,361,240, issued to the inventor of this application, discloses a pressure-compensated PVDF hydrophone that contains a hollow mandrel having a concavity at an outer surface. A flexible and resilient piezoelectric film, preferably made from a PVDF material, is wrapped several times around the mandrel over the concavity to act as the active element of the hydrophone. The volume between the surface of the inner layer of the film and the concavity provides a pressure compression chamber. This hydrophone has been found to be responsive to varying hydrodynamic pressure fields but is substantially insensitive to acceleration forces, localized impacts and variations in hydrostatic pressures.
To perform seismic surveys in water-covered areas, one or more arrays of hydrophones, each array having a plurality of serially coupled hydrophones, are deployed on the bottom of a water-covered area or are towed behind a vessel. In bottom cable applications, hydrophones are commonly built as an integral part of the cable. Each hydrophone is hermetically sealed with a suitable material, such as polyurethane. Such cable constructions are not conducive to easy repairs in the field. Defective hydrophone sections are removed and a cable section containing a working hydrophone is spliced in the place of the defective hydrophone. Such repairs are usually less reliable than unitary constructions and require excessive repair time, which can significantly increase the cost of the surveying operations, especially when performing three-dimensional seismic surveys as the down time can cost several thousand dollars per hour. Thus, there has been an unfilled need to provide a hydrophone which is easy to assemble into a hydrophone cable and easy to repair in the field.
The present invention addresses the above-noted problems and provides a segmented hydrophone that preferably utilizes a flexible and resilient piezoelectric material and a method of making same. The hydrophone segments may be combined to form a single hydrophone. The hydrophone segments removably attach to the cable which is suitably configured to accommodate the hydrophone segments. Any hydrophone segment can readily be replaced without requiring any splicing of the cable. The hydrophone is responsive to varying hydrodynamic pressure fields, but is substantially inert to acceleration forces, localized impacts and variations in hydrostatic pressure. The hydrophone of the present invention also provides an output response that substantially reduces unwanted strain response and noise due to the coupled stresses of the cable.